The present invention relates generally to drill bits used to bore through earthen formations. More specifically, the present invention relates to steel bodied drill bits and the methods employed in securing polycrystalline diamond compact (PDC) cutters to such bits.
Steel bodied bits customarily employ PDC cutters that are secured to the bit by mounting the cutter in a receptacle or pocket formed in the bit body and brazing the cutter into the steel pocket. Hardfacing material such as tungsten carbide is also applied to the steel body of the bit to strengthen the bit and reduce bit body wear. The hardfacing material is applied in a layer by heating the material to its liquid or molten state with an oxyacetylene torch or other suitable means to bond the hardfacing material to the steel of the bit body. The high temperatures required to apply the hardfacing material to the steel bodied bit are damaging to the diamond portion of the PDC cutter so that the cutter must be secured to the bit after the hardfacing step has been completed.
It is undesirable to form the pocket for the cutter after the hardfacing material has been applied because of the difficulty in machining or cutting through the extremely hard hardfacing material. Conventionally, a cutter pocket is formed in the steel bit body before the hardfacing is applied, and a graphite or silicate displacement plug is temporarily used to occupy the pocket during the application of the hardfacing material. After the application of the hardfacing material, the displacement plug is ground out or otherwise removed from the pocket. One of the problems with the described technique is that the molten hardfacing material does not bond to the graphite or silicate displacement plug and can in fact shrink away from the material of the plug so that a gap is left between the plug body and the surrounding hardfacing material. Additionally, where several cutters are closely spaced, the area between the adjacent cutters may not be sufficiently large to permit deposition of the molten hardfacing material due, in part, to the shrinkage characteristic of the hardfacing material. The high temperatures can also melt the relatively thin web of steel between closely spaced sockets. In either case, when the cutter is inserted into the pocket, a gap exists between the body of the cutter and the surrounding hardfacing. This gap may be filled with a brazing material, however, the erosion resistance of the brazing material is not as good as that of the hardfacing material. During use of the bit in drilling the high pressure drilling fluids and entraining abrasives in the fluid erode the brazing material or steel in the gap between the cutter body and the hardfacing material, eventually leading to loosening and even loss of the cutter.
The displacement plug is coated with a thin layer of material such as a refractory metal that acts as a wetting agent allowing the molten hardfacing material to adhere to the displacement plug. When the plug is removed, the cured hardfacing material remains disposed immediately adjacent the cutter pocket and has opening dimensions over the cutter pocket that substantially conform to the dimensions of the cutter. The hardfacing material and steel defining the cutter pocket within the bit body cooperate to form a deeper composite pocket that has a significantly reduced gap between the hardfacing material and the cutter body. The small areas between adjacent cutters are also filled with the hardfacing material and provide a similar composite pocket that closely surrounds the cutter.
The method of the present invention reduces the area exposed to erosion around the cutter and further provides the additional structural support of hardfacing material against the cutter body to enhance the structural strength of the connection between the cutter and the steel bit.
From the foregoing it will be appreciated that a primary object of the present invention is to provide a method for extending the hardfacing material employed on a steel bodied bit to the edges of the pocket holding a PDC cutter to minimize the gap between the hardfacing material and the cutter and to strengthen the structural engagement between the cutter and the bit body.
Another object of the present invention is to provide a bit in which the gap between the cutter body and the surrounding hardfacing material of the bit is reduced to a minimum to prevent the erosion of softer material in the gap between the hardfacing material and the cutter body.
Yet another object of the present invention is to provide an inexpensive method for forming a bit having a hardfacing layer that includes a cutter receiving opening having close dimensional tolerance with a cutter to be received in the opening without the need for machining or milling through the hardfacing material.
Another object of the present invention is to provide a means for maintaining a close tolerance opening for receiving a cutter in a steel bit pocket wherein high temperatures are employed in the application of hardfacing material to the surface of the steel bodied bit.
A specific object of the present invention is to provide a method in which a refractory metal is employed to coat a disposable displacement plug wherein the refractory metal will act as a wetting agent to combine with hardfacing material applied at high temperatures to secure the hardfacing material to the displacement member. The cured hardfacing is left at a location that closely approximates the external dimensions of the displacement member exposed from the steel pocket to thereby provide a composite receptacle or pocket for receiving a cutter on a steel bodied bit with a minimum of space between the body of the cutter and the opening through the hardfacing material.
The foregoing objects, features and advantages of the invention as well as others will be more readily understood and appreciated by reference to the following drawings, specifications and claims.